1. Field of the Invention.
This invention relates to a measurement device called impedancemeter for measuring the surface impedance of a material.
More particularly, the invention deals with materials that are very good conductors electrically, presented in the form of a thin sheet, such as the conductive metals or compound materials, e.g. with carbon fiber, used in aeronautic equipment. These materials can, if necessary, be covered with paints or insulation coatings.
2. State of the Prior Art.
It is recalled that the surface impedance Z.sub.S characterizing such materials results from the following definition. The material carries a surface current density J.sub.s which creates a potential difference per unit of length represented by the longitudinal electrical field E.sub.tg. The impedance Z.sub.S is defined by the relation: E.sub.tg =Z.sub.s J.sub.s.
In the case of the material being conductive, the impedance Z.sub.S is related to the electrical conductivity .sigma. and to the thickness d of the material by the relation Z.sub.S =1/(.sigma. d) as long as there is no occurrence of skin effect. If we consider a square plate of impedance Z.sub.S carrying a current I, a potential difference V appears at the terminals of the plate such that V=Z.sub.S I.
The preceding remark can be turned to good account to measure the surface impedance Z.sub.S directly. Nevertheless, with a very conductive material, the impedance of the material is in series with the impedance of the wires that inject the current I. It is then no longer possible to distinguish the contribution of the material and that of the measuring circuit in the measuring of the potential difference V, which then becomes impossible. Moreover, such measurement cannot be carried out if the material is covered with non-conductive material.
To obviate this drawback, it has already been proposed that the impedance Z.sub.S be measured without any electrical contact between an impedancemeter and the material.
A known impedancemeter further comprises a first antenna, called transmitting antenna, that creates a given electromagnetic field, and a second antenna, called receiving antenna. A measurement chain connected to the second antenna enables the electromagnetic field to be measured in a given frequency band. During a first stage, the transmitting antenna is placed at a predetermined constant distance from the receiving antenna and is moved away from material likely to perturb the field emitted directly by the first antenna and received by the second antenna. During a second stage, a sample of the material to be tested is interposed between the transmitting and receiving antennae. Throughout the two stages, an alternating voltage wobbled in a predetermined frequency range is applied to the transmitting antenna. The voltage measurements at the terminals of the receiving antenna for the two stages are analyzed and interpreted in order to determine the surface impedance value Z.sub.S of the material sample.
The material usually fills a two-dimensional opening in a metallic panel of large dimensions. Each of the antennae is comprised of a conductive circular loop attached to an insulating holder plate on both sides of the opening plane.
The transmitting antenna creates a magnetic field that is uniform with the area surrounding the material parallel to the plane of the material. During the first stage, the receiving antenna receives a magnetic field from the other side of the opening at a given distance. During the second stage, the receiving antenna receives a magnetic field that is attenuated by the presence of the material in the opening.
In the case of a circular opening of radius a, the ratio of the measured voltages V.sub.o and V between the terminals of the receiving antenna during the first and second stages satisfy the approximate relation: EQU V/V.sub.o =1/(1+jf/f.sub.c)
with f.sub.c =(3Z.sub.S /(8.mu..sub.o a)) (1+2.pi. R.sub.c /Z.sub.S)
whereby Z.sub.S is the surface impedance of the material, a is the radius of the opening, R.sub.c is the possible resistance of the joint connecting the material and the metallic panel, and .mu..sub.O =4.pi..10.sup.-7 H/m is the permeability of vacuum.
If the electrical contact between the material and the metallic panel in the opening is perfect, i.e. if R.sub.c =0, we obtain f.sub.CO =3Z.sub.S /8.mu..sub.o a.
In this way, the fixing of the material in the opening provokes in the voltage picked up by the receiving antenna an attenuation or transfer function of the "first-order low-pass filter" type characterized by a cut-off frequency f.sub.CO proportional to the surface impedance Z.sub.S of the material.
The known impedancemeter with two completely separate antennae and the measurement method inherent in this device mainly have the three following drawbacks:
i) The expression of f.sub.C shows that if the material-conductive panel contact is not perfect, the frequency f.sub.C is shifted with regard to f.sub.CO. As the value of R.sub.c cannot be quantified, an assembly must be set up to attempt to minimize the interference resistance R.sub.c. The value of the latter is then neglected in the interpretation of the measurement, without it being possible to evaluate the error made. Furthermore, a sample material covered with paint cannot be measured as the contact is bad.
ii) The frequency f.sub.c depends on the radius a of the opening, i.e. on the dimension of the sample material tested. Only materials cut out to a given format so as to fix them in the opening can be tested by this method. It is therefore necessary to cut out a sample of each material to be tested, and a material belonging to a complex structure cannot be tested.
iii) The distance between the transmitting antenna and the material must be sufficient to obtain a magnetic field that is uniform with the area surrounding the material. As a result, the distance between the antennae is relatively long and the voltage measured by the receiving antenna is relatively low.